Plasma etching is frequently used in a semiconductor manufacturing process. In the plasma etching, ions are accelerated by an electric field in order to etch an exposed surface on a substrate. The electric field is generated according to high frequency signals generated by a high frequency generator of a high frequency power system. The high frequency signals generated by the high frequency generator need to be precisely controlled such that the plasma etching is efficiently performed.
The high frequency power system may include the high frequency generator, an impedance matcher, and a plasma chamber. The high frequency signals are used to drive loads in order to manufacture various components such as integrated circuits (ICs), solar panels, compact disks (CDs), and DVDs.
The high frequency signals are received in the impedance matcher. The impedance matcher matches an input impedance of the impedance matcher to a characteristic impedance of a transmission line between the high frequency generator and the impedance matcher. The impedance matching helps to minimize an amount of power (“forward power”) of the impedance matcher applied to a resonant network toward the plasma chamber in a forward direction, and helps to minimize an amount of power (“reverse power”) reflected from the impedance matcher to the high frequency generator. When the input impedance of the impedance matcher matches the characteristic impedance of the transmission line, output of the forward power from the high frequency generator to the plasma chamber may be maximized and the reverse power may be minimized.
In supplying the high frequency power, there are two general methods of applying a high frequency signal to a load. First, a traditional method is to apply a continuous wave signal to the load. In the continuous wave mode, the continuous wave signal is generally a sinusoidal wave continuously outputted from a power supply to the load. In the continuous wave method, the high frequency signal is assumed as sinusoidal output, and the amplitude and/or frequency of the sinusoidal wave may be changed in order to change output power applied to the load.
In another method of applying the high frequency signal to the load, a pulse type high frequency signal is used. In the pulse operation mode, a high frequency sinusoidal signal is modulated by a modulation signal in order to define an envelope for a modulated sinusoidal signal. In the related pulse modulation scheme, the high frequency sinusoidal signal is typically outputted at constant frequency and amplitude. Power transmitted to the load does not change the sinusoidal wave or the high frequency signal, but is changed by changing the modulation signal.
In a general high frequency power supply configuration, output power applied to the load is decided using a sensor that measures forward power and reflected power of the high frequency signal applied to the load or measures a voltage and a current. One set of these signals is analyzed in a general feedback loop. Such analysis generally decides a value of power used to adjust the output of the high frequency power supply in order to change power applied to the load. In a high frequency power transmission system in which the load is a plasma chamber, a change in a load impedance causes corresponding variable power applied to the load because applied power is partially a function of the load impedance.
Furthermore, transition from a continuous wave high frequency power transmission system to a pulse high frequency power transmission system presents additional problems. In a typical plasma system, power consumed by plasma depends on the impedance of the plasma. When the impedance is changed to a time scale of a high frequency pulse (for example, a range of 1 kHz to 10 kHz), a sensor and an actuator in an impedance matcher and a high frequency generator need to react with a similar time scale to provide optimal power coupling to the plasma load in order to keep the plasma between pulses. Furthermore, the time response of the impedance is plasma-dependent and changes according to factors such as chemical materials, pressure, and power coupling. Furthermore, various parasitic elements outside the plasma such as resistance loss in a high frequency coupling antenna or a matching system represent power coupling efficiency changing over time during a pulse period, and this is because the parasitic elements are a constantly consumed impedance serially coupled to the impedance load changing over time. Furthermore, since the transmission and reflected power sensors and the high frequency generator are generally corrected for matched terminations, power compensation due to impedance mismatch may contribute to an increase in variability in power transmission.
Furthermore, in order to minimize an influence of impedance transients, it is important to achieve process synchronization between frequency measurement and prediction of a corresponding position of the impedance actuator in the impedance matcher. Furthermore, it becomes more difficult to realize process reproducibility and anti-attributes when achieving a target frequency.
In a current high frequency power generation system, in order to achieve impedance matching between a high frequency generator and a load, a frequency of a high frequency signal may be adjusted within a predetermined range based on a selected target frequency or center frequency. Such frequency-based impedance adjustment is called radio frequency tuning (RFT). In some RFT configurations, the frequency of the high frequency signal may be adjusted toward a predetermined range of limits.